1. Field of the Invention
This invention relates to a stepping motor, and more particularly to a stepping motor for driving a diaphragm unit forming a diaphragm aperture by a plurality of diaphragm blades.
2. Related Background Art
Numerous so-called stepping motors in which pole teeth are disposed on the outer periphery of a rotor and said pole teeth are magnetized to step-drive the rotor about the axis of the rotary shaft thereof are known including those disclosed in U.S. Pat. Nos. 3,014,141, 3,549,918, 3,866,104, 4,174,485 and 4,381,465. Also some of the inventors concerned in the present invention filed in Japan a plurality of applications including Patent Application No. 61-85516 and Utility Model Application No. 61-55846 for a diaphragm driving device having a stepping motor as a drive source, and these Japanese applications were filed as U.S. application Ser. No. 035,453.
The diaphragm device of the aforementioned U.S. application Ser. No. 035,453 (filed in U.S. on Apr. 7, 1987) is designed such that the opening-closing operation of the diaphragm blades is accomplished by a stepping motor.
A plurality of diaphragm blades are combined and are adapted to be opened and closed to form a diaphragm aperture.
The diaphragm aperture has a relation to the exposure value of a camera, and the exposure value of the camera is determined by various types of information such as shutter speed, diaphragm aperture, brightness of an object and film speed, and is determined by said information being input to an exposure control device in the camera.
The diaphragm aperture has its aperture value determined by said exposure control device, and the diaphragm blades are operated to an aperture corresponding to the aperture value by the stepping motor and are stopped at the aperture corresponding to the aperture value.
On the other hand, the stepping motor has a stator provided with pole teeth arranged arcuately around a rotor having N (North) poles and S poles magnetized on the surface thereof. When the supply of electric power to the coil of the motor stopped, the rotor is stopped at a magnetically stable position by the magnetic attraction between the magnetic poles of the rotor and corresponding pole teeth of the stator.
When the supply of electric power to the stepping motor is stopped after the stepping motor is driven to the aforementioned aperture value calculated by the exposure calculating device to form the aperture of the diaphragm blades, the rotor moves to the aforementioned magnetically stable position. As a result, the diaphragm blades moved in response to the movement of the rotor and the aperture of the diaphragm blades varies. Therefore, the calculated aperture value and the aperture value of the aperture formed by the diaphragm blades become more or less different from each other and an error of exposure control arises.